A vehicle air bag module is typically placed in a vehicle passenger compartment to deploy an air bag during a collision and thereby cushion a vehicle passenger from impact with vehicle structures. An air bag module generally includes a container enclosing a folded air bag and a gas generator or inflator. At the onset of a collision, the gas generator produces an inert gas (e.g., nitrogen) which is directed under pressure against the air bag to force the air bag out of the container and into the passenger compartment.
When the module is assembled, the air bag is folded in a predetermined manner in order to enable the air bag to be effectively deployed during a collision. Thus, once the air bag is properly folded in the module, its position within the module should not be disturbed. In an attempt to avoid disturbing a folded air bag, assembly of the module has typically included installation of the inflator as a first phase of assembly of the module, and installation of the air bag as the last phase of assembly of the module.
There are, however, disadvantages to installing a live inflator in a module during the first phase of assembly of a module. For example, live inflators generally include volatile propellants, pyrotechnic ignition compounds, and initiators, which can be ignited by vibration, friction, heat, shock, or other sources of energy. The more the live inflators are handled during assembly, the higher the risk of accidental ignition. Installation of live inflators during the first phase of assembly of a module results in increased handling of inflators during subsequent assembly of the air bag with the module.
In addition, modules are often transported and stored prior to assembly in a vehicle. When a live inflator is installed during the first phase of assembly of a module, the entire module, including the live inflator, must be shipped and stored as a complete unit. In the event of accidental ignition of the inflator during shipping or storage, the entire module, including the air bag and container, is likely to be damaged.
To reduce the risk of accidental ignition, it is desirable to reduce the amount of handling of the inflators prior to installation in a vehicle. Such a reduction can be obtained by installing the live inflators as the final step of module assembly. Installing the inflators during final assembly also permits the inflators to be stored and shipped separately from the other portions of the modules. Thus, if accidental ignition occurs during storage or shipment, the other portions of the module will not be damaged.
Some air bag module designs have reflected concern over the possibility of accidental ignition. U.S. Pat. No. 4,153,273, for example, provides a diffuser secured on the inside of an air bag, and locates the air bag and the diffuser within a container. A dummy gas generator is installed in the diffuser during initial assembly of the module. A live gas generator is installed later during the final assembly stage when the module is completed.